1. Field of the Invention
This invention relates generally to disk drives, and more particularly to a magnetic recording disk drive that includes a system for substantially suppressing a repetitive or periodic disturbance where the frequency of the disturbance is neither known nor constant.
2. Description of the Related Art
Magnetic recording hard disk drives (HDDs) use an actuator, typically a rotary voice-coil-motor (VCM) actuator, for positioning the read/write heads on the data tracks of the recording disks. The HDD has a servo control system that receives a position error signal (PES) from servo positioning information read by the heads from the data tracks and generates a VCM control signal to maintain the heads on track (track “following”) and move them to the desired track (track “seeking”) for reading and writing of data.
HDDs experience disturbance forces during normal operation that affect the positioning of the heads on the data tracks. Because HDDs are used in a variety of environments they are also subject to a wide range of external disturbances. For example, in portable music players and audio/video devices that use HDDs, the external disturbance may come from walking or jogging, or from automobile motion. These disturbances are typically low-frequency vibrations that are periodic, i.e., repetitive at regular or predictable intervals, but the frequency is not known. Also, these periodic disturbances typically have a fundamental frequency that is not constant but slowly changing.
There are many known techniques for control of one particular type of periodic disturbance in HDDs, i.e., repeatable disk runout (RRO). However, RRO occurs at the known frequency of disk rotation so there is no need to identify the frequency and no need to address the problem of a periodic disturbance with a changing frequency. The conventional approach to control of RRO and other periodic disturbances, such as the use of a repetitive controller in the servo control system feedback loop, does not work for mobile HDD applications because it requires that the frequency be known and constant.
In addition to RRO, there are other HDD disturbances that exhibit narrow-band frequency characteristics. To compensate for narrow-band disturbances, peak filters are often used in the HDD servo control system, if the frequency of the disturbance is known. However, the frequency of the disturbance may not be known and/or may vary with time. Thus HDDs may have one or more types of disturbance sensors, such as accelerometers and rotational vibration sensors, that provide measurements of the disturbances. The output of these types of sensors is often used for suspending operation of the HDD until the disturbance has subsided, or input to a feedforward controller to create a control signal that compensates for the head's off-track motion caused by the disturbance. However, the output of these sensors can also be used to identify the frequency of the disturbance. In co-pending application Ser. No. 11/349,774 filed Feb. 7, 2006 and assigned to the same assignee as this application, an HDD is described that uses a vibration sensor and an adjustable peak filter coupled to the servo control system. The frequency of the vibration is identified from the sensor output and used to adjust the peak filter frequency up or down, so as to suppress disturbances over a relatively wide range of frequencies.
What is needed is a HDD that includes a system without a vibration sensor to substantially suppress a repetitive or periodic disturbance where the frequency of the disturbance is neither known nor constant.